1. Field of the Invention
The present invention relates to a DC motor and a full bridge circuit, and more particularly, to a DC motor and a full bridge circuit utilizing operational amplifiers and multiplexers to fix output voltages and avoid reverse current.
2. Description of the Prior Art
A DC motor driver is a necessary power transformation device in modern industries and the information society. The DC motor is capable of transforming electricity into kinetic energy required for driving devices. Conventional motors include DC motors, AC motors, and stepping motors. DC motors and AC motors are often applied in products not requiring delicate manipulations. For example, blades of an electric fan are rotated with a DC motor or an AC motor. As the technology of digital products grows, a rotation rate of a DC motor or an AC motor is required to be faster and faster. However, with a high rotation rate of a motor, the current of the motor cannot be consumed completely. The unconsumed and therefore remaining currents reversely flow to a corresponding power supply. This scenario leads to damages of controllers and drivers of the motor. Therefore, the damages caused by reverse current under a high rotation rate of the motor have to be avoided.
Please refer to FIG. 1, which is a diagram of a DC motor 10 in the prior art. The DC motor 10 comprises a power supply 12, an input capacitor C1, a hall sensor 16, a first controller 20, a second controller 21, and a full bridge circuit 14. The power supply 12 is utilized for generating an input voltage Vin. The input capacitor C1 is coupled to the power supply 12. A voltage difference between both terminals of the input capacitor C1 is a supply voltage VDD. The Hall sensor 16 has a first output end 162 for generating a first timing control signal H+, and a second output end 164 for generating a second timing control signal H−.
The first controller 20 has a first input end 102 coupled to the first output end 162 of the Hall sensor 16, a second input end 104 coupled to the second output end 164 of the Hall sensor 16, a first output end 106 for generating a first switch control signal, and a second output end 108 for generating a second switch control signal. The second controller 21 has a first input end 112 coupled to the second output end 164 of the Hall sensor 16, a second input end 114 coupled to the first output end 162 of the Hall sensor 16, a first output end 116 for generating a third switch control signal, and a second output end 118 for generating a fourth switch control signal.
The full bridge circuit 14 has an input end 142 coupled to the power supply 12 and the input capacitor C1, and the voltage at the input end 142 is the supply voltage VDD. The full bridge circuit 14 comprises a first switch SW1, a second switch SW2, a third switch SW3, a fourth switch SW4, and a motor loading Le. The first switch SW1 has a control terminal 132 coupled to the first output end 106 of the first controller 20, an input end 134 coupled to the power supply 12 and the input capacitor C1, and an output end 136 for generating a first output voltage Vout1. The second switch SW2 has a control terminal 152 coupled to the second output end 108 of the first controller 20, an input end 154 coupled to ground, and an output end 156 coupled to the output end 136 of the first switch SW1. The third switch SW3 has a control terminal 172 coupled to the first output end 116 of the second controller 21, an output end 174 coupled to the power supply 12 and the input capacitor C1, and an output end 176 for generating a second output voltage Vout2. The fourth switch SW4 has a control terminal 192 coupled to the second output end 118 of the second controller 21, an input end 194 coupled to ground, and an output end 196 coupled to the output end 176 of the third switch SW3. The motor loading Le has a first terminal 182 coupled to the first switch SW1 and the second switch SW2, and a second terminal 184 coupled to the third switch SW3 and the fourth switch SW4.
The first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 may be metal-oxide semiconductor transistors, for example, the first switch SW1 and the third switch SW3 are P-type metal-oxide semiconductor transistors, and the second switch SW2 and the fourth switch SW4 are N-type metal-oxide semiconductor transistors. The first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 may also be bipolar-junction transistors, for example, the first switch SW1 and the third switch SW3 are NPN bipolar-junction transistors, and the second switch SW2 and the fourth switch SW4 are PNP bipolar-junction transistors. In the same way, the first switch SW1 and the third switch SW3 are PNP bipolar-junction transistors, and the second switch SW2 and the fourth switch SW4 are NPN bipolar-junction transistors. Even the four switches SW1-SW4 are all PNP bipolar-junction transistors or NPN bipolar-junction transistors.
Please refer to FIG. 1 and FIG. 2. FIG. 2 is a timing diagram of the signals shown in FIG. 1. The DC motor 10 controls the switches of the full bridge circuit 14 and the waveforms of the first output voltage Vout1 and the second output voltage Vout2 are square waves. The first timing control signal H+ and the second timing control signal H− outputted from the Hall sensor 16 control turning on and turning off the first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4. The first switch SW1 and the fourth switch SW4 are turned on and the third switch SW3 and the second switch SW2 are turned off, when the first timing control signal H+ is low and the second timing control signal H− is high. The direction of a motor current ILe flows from the first output voltage Vout1 to the second output voltage Vout2. At this time, the first output voltage Vout1 is high and the second output voltage Vout2 is low. During the transition of the first timing control signal H+ and the second timing control signal H−, the second switch SW2 and the fourth switch SW4 are turned on, and the first switch SW1 and the third switch SW3 are turned off. The motor current ILe weakens gradually through the second switch SW2 and the fourth switch SW4. At this time, both the first output voltage Vout1 and the second output voltage Vout2 are low. When the first timing control signal H+ is high and the second timing control signal H− is low, the second switch SW2 and the third switch SW3 are turned on, and the first switch SW1 and the fourth switch SW4 are turned off. The motor current ILe flows from the second output voltage Vout2 to the first output voltage Vout1, at this time, the first output voltage Vout1 is low, and the second output voltage Vout2 is high.
Please refer to FIG. 3, which is a waveform diagram of the signals of FIG. 1 when a high rotation rate of the DC motor results in a voltage spike. During the first stage, the first timing control signal H+ is low, and the second timing control signal H− is high. The motor current ILe flows from the first output voltage Vout1 to the second output voltage Vout2, at this time, the first output voltage Vout1 is high, and the second output voltage Vout2 is low. During the second stage and the transition of the first timing control signal H+ and the second timing control signal H−, the motor current ILe weakens gradually through the second switch SW2 and the fourth switch SW4. However, since the rotation rate of the DC motor is high, the motor current ILe cannot be weakened to be zero after the transition of the switches. Therefore, during the third stage, the motor current ILe flows reversely to the supply voltage VDD through the second switch SW2 and the fourth switch SW4, and charges the input capacitor to result in a voltage spike. As shown in FIG. 3, the magnitude of the voltage spike depends on the magnitude of the reverse current flowing into the input capacitor C1 and the capacitance of the input capacitor C1. During the fourth stage when the first timing control signal H+ turns to high and the second timing control signal H− turns to low, the motor current ILe flows from the second output voltage Vout2 to the first output voltage Vout1, at this time, the first output voltage Vout1 is low, and the second output voltage Vout2 is high.
Please refer to FIG. 4, which is a diagram illustrating the flow of the motor current ILe during the first stage shown in FIG. 3. During the first stage, the first switch SW1 and the fourth switch SW4 are fully on. The motor current ILe flows from the first output voltage Vout1 to the second output voltage Vout2.
Please refer to FIG. 5, which is a diagram illustrating the flow of the motor current ILe during the second stage shown in FIG. 3. During the second stage, the second switch SW2 and the fourth switch SW4 are turned on. At this time, the motor current ILe weakens gradually. The motor current ILe continues to flow from the first output voltage Vout1 to the second output voltage Vout2.
Please refer to FIG. 6, which is a diagram illustrating the flow of the motor current ILe during the third stage shown in FIG. 3. During the third stage, motor current ILe continues to flow from the first output voltage Vout1 to the second output voltage Vout2 due to the motor current ILe not decaying to zero yet. The second switch SW2 and the third switch SW3 are turned on, and the motor current ILe flows from the second switch SW2 to the third switch SW3. At this time, the motor current ILe charges the input capacitor C1 to increase the supply voltage VDD and to result in a voltage spike.
Please refer to FIG. 7, which is a diagram illustrating the flow of the motor current ILe during the fourth stage shown in FIG. 3. During the fourth stage, since the motor current ILe has weakened to be zero, the second switch SW2 and the third switch SW3 are turned on whereas the first switch SW1 and the fourth switch SW4 are turned off. The motor current ILe flows from the second output voltage Vout2 to the first output voltage Vout1.
For a DC motor having a low rotation rate, the motor current ILe can decay to zero during the transition of the first timing control signal H+ and the second timing control signal H−. However, in modern applications, the rotation rate of a modern motor is ever increasing. When the rotation rate of the motor exceeds a limit, the motor current ILe has not weakened to be zero after the transition of the switches. At this time, the motor current ILe flows reversely to the supply voltage VDD through the second switch SW2 and the third switch SW3, and charges the input capacitor C1 to result in the voltage spike. Therefore, the controller and the driver of the DC motor 10 would be damaged or burned down, the power supply 12 may be burned down also, and the reliability and the effective operational range of the system of the DC motor 10 would be degraded.